Q20. In the reaction given below, choose the condition that gives an anti-Markovnikov’s
product.
(A) Peroxide / HCI
(B) Aqueous mercuric acetate treatment
(C) Diborane addition
(D) Sulfuric acid addition
Anti-Markovnikov Addition to Alkenes: Peroxide Effect with HBr
The correct condition for anti-Markovnikov addition of HCl to an alkene is option (A) peroxide.
Option Analysis
Option (A), peroxide, triggers a free radical mechanism for HX addition where X adds to the less substituted carbon, yielding the anti-Markovnikov product specifically with HBr (though question specifies HCl). This occurs via peroxide-initiated Br- addition to the less substituted carbon, forming the more stable secondary radical, followed by H-abstraction.
Option (B), aqueous mercuric acetate, refers to oxymercuration for alkene hydration, which follows Markovnikov regioselectivity (OH on more substituted carbon) without carbocation rearrangements.
Option (C), diborane addition (hydroboration), adds BH2 anti-Markovnikov to the less substituted carbon (syn addition); subsequent H2O2/OH- oxidation replaces B with OH retention, giving anti-Markovnikov alcohol.
Option (D), sulfuric acid, catalyzes acid hydration forming Markovnikov alcohol via carbocation on the more substituted carbon, prone to rearrangements.
The anti-Markovnikov product in alkene reactions reverses typical addition, placing hydrogen on the more substituted carbon—key for organic synthesis and CSIR NET life sciences.
Peroxide Effect Mechanism
Peroxides initiate free radical addition of HBr: ROOR homolyzes to RO- , abstracting Br from HBr to form Br- ; Br- adds anti-Markovnikov, yielding stable radical; radical takes H from HBr, regenerating Br- . Unique to HBr due to bond strengths; HCl/HI ineffective.
Comparing Addition Methods
| Method | Regioselectivity | Product | Mechanism |
|---|---|---|---|
| HBr + Peroxide | Anti-Markovnikov | Alkyl bromide | Free radical |
| Diborane (BH3) + Oxid. | Anti-Markovnikov | Alcohol | Syn hydroboration |
| Hg(OAc)2/H2O | Markovnikov | Alcohol | Mercurinium ion |
| H2SO4/H2O | Markovnikov | Alcohol | Carbocation |
CSIR NET Relevance
Mastering anti-Markovnikov vs Markovnikov distinguishes electrophilic from radical pathways, vital for predicting products in alkenes. Hydroboration offers syn stereochemistry, unlike radical anti-Markovnikov.
